This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-028553, filed Feb. 5, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to a method of correcting a photomask used for the manufacture of a semiconductor integrated circuit, and more particularly to a method of correcting a photomask by applying a focused ion beam (FIB) to a defect of the photomask to repair the defect.
2. Description of the Related Art
In correcting a defect of a photomask using a focused ion beam, one point drift correction is used in order to totally correct a drift of an image due to charge up, or a drift due to thermal expansion of a mask or a mask holder. In the one point drift correction, a pinhole is formed in a pattern of the photomask. The beam is moved to the portion to be corrected by using the pinhole as a reference point, thereby enhancing correction accuracy. A specific example of the one point drift correction will be described below with reference to FIGS. 8 to 10.
A focused ion beam 22a is applied to a part of a pattern 12 which is formed on a substrate 11 for a photomask, so that a pinhole 13, which serves as a reference point, is formed. Then, an ion beam 22b is applied from an ion beam source 21 to a scan region 14 including the pinhole 13. Secondary ions (secondary charged particles) 23 released from the portion where the pinhole 13 is formed are detected by a detector 24, thereby recognizing (obtaining) the reference point. Then, the positional relationship between the detected position of the pinhole 13 (position of the reference point) and the portion to be corrected which is known in advance, is calculated. After re-confirming the position of the pinhole 13, the ion beam is applied to the portion to be corrected, thereby correcting the defect. After that, by repeatedly performing the ion beam irradiation for confirming the reference point and the ion beam irradiation for correcting the defect, the defect correction is completed. In this way, by confirming the reference point immediately before the defect correction processing, the influence of a drift can be reduced to a minimum.
If defect correction is performed using the above-described one point drift correction, it is conventionally considered that a round shape is ideal as the shape of a planar pattern of the reference point (pinhole) (refer, for example, to Jpn. Pat. Appln. KOKOKU Publication No. 5-4660). From this point of view, the pinhole is conventionally formed by applying the ion beam to a square-shaped area with an equal number of dots in longitudinal and horizontal directions (e.g. 4xc3x974 dots).
However, if a phase shift mask is used as a photomask, a proper phase shift operation (phase effect) cannot be obtained at the region where the pinhole is formed. Therefore, if the pinhole is large relative to the pattern, a region which affects a projected image is also large. If the pinhole is large, the scan region for detecting the pinhole needs to be large. Thus, the beam scan damages a wide area of the pattern. Therefore, if the pattern is small, the line size of the projected image is small, which is a problem. Further, in the case of correcting a pinhole mark, since a carbon film deposited for correction does not have a phase shift operation, the smaller the pattern, the wider the region which affects the projected image.
As shown in FIG. 10, in general, a detector 24 is disposed obliquely above the pinhole 13. Thus, if the pattern of the pinhole has a square shape, the following problem will arise. Since the detector 24 is disposed obliquely above the pinhole 13, the secondary ions 23 coming out of the pinhole 13 are considered to be blocked to some extent by a sidewall at the detector 24 side (the left sidewall in FIG. 10). Therefore, the length of the pinhole 13 in the X direction shown in FIG. 10 needs to be increased to a degree. In this case, if the pinhole 13 is a square pattern, the length of the pinhole 13 in the Y direction is equal to that in X direction. Regarding the X direction, the arrival of the secondary ions to the detector 24 is limited by the left sidewall (in FIG. 10). Thus, the yield of the secondary ions is biased to the right region (in FIG. 10), and a certain degree of positional accuracy can be obtained. However, since such a situation will hardly occur in the Y direction, the positional accuracy (recognition accuracy) of the pinhole in the Y direction cannot sufficiently be obtained. In actuality, as shown in FIG. 11, a secondary ion image 31 corresponding to the pinhole is a vertically elongated image in the Y direction.
Further, in the prior art, many secondary ions are blocked by a sidewall on the detector 24 side as described above. Thus, there is a problem that the secondary ions from the bottom portion of the pinhole, i.e., the exposed surface of the substrate, do not reach the detector with efficiency.
In the conventional method of correcting the photomask using one point drift correction, if the phase shift mask is used as a photomask, the region, where phase shift operation (phase effect) cannot be obtained, increases. Thus, a proper projected image cannot be obtained, which is a problem. There are also problems that the positional accuracy (recognition accuracy) of the reference pinhole is insufficient, and that the secondary ions (secondary charged particles) from the pinhole cannot efficiently be made incident on the detector.
According to a first aspect of the present invention, there is provided a method of correcting a photomask, comprising: preparing a photomask substrate with a mask pattern including a phase shift pattern; forming a reference hole by removing a part of the mask pattern; applying an ion beam from an ion beam source to an area including the reference hole to allow secondary charged particles to be released from the reference hole; obtaining a position of the reference hole by detecting the secondary charged particles by a detector; calculating a positional relationship between the obtained position of the reference hole and a position of a defect of the mask pattern; and correcting the defect by applying an ion beam from the ion beam source to the defect, based on the calculated positional relationship, wherein a pattern of the reference hole, as viewed in a direction perpendicular to a top surface of the photomask substrate, is substantially rectangular, and a longitudinal direction of the rectangular pattern is parallel to a longitudinal direction of the phase shift pattern.
According to a second aspect of the present invention, there is provided a method of correcting a photomask, comprising: preparing a photomask substrate having a mask pattern; forming a reference hole by removing a part of the mask pattern; applying an ion beam from an ion beam source to an area including the reference hole to allow secondary charged particles to be released from the reference hole; obtaining a position of the reference hole by detecting the secondary charged particles by a detector, a charged particle incidence portion of the detector being disposed obliquely above the reference hole; calculating a positional relationship between the obtained position of the reference hole and a position of a defect of the mask pattern; and correcting the defect by applying an ion beam from the ion beam source to the defect, based on the calculated positional relationship, wherein a pattern of the reference hole, as viewed in a direction perpendicular to a top surface of the photomask substrate, is substantially rectangular, and a longitudinal direction of the rectangular pattern is parallel to a straight line obtained by projecting, on the top surface of the photomask substrate, a straight line linking the incidence portion of the detector and the reference hole.
According to a third aspect of the present invention, there is provided a method of correcting a photomask, comprising: preparing a photomask substrate having a mask pattern; forming a reference hole by removing a part of the mask pattern; applying an ion beam from an ion beam source to an area including the reference hole to allow secondary charged particles to be released from the reference hole; obtaining a position of the reference hole by detecting the secondary charged particles by a detector, a charged particle incidence portion of the detector being disposed obliquely above the reference hole; calculating a positional relationship between the obtained position of the reference hole and a position of a defect of the mask pattern; and correcting the defect by applying an ion beam from the ion beam source to the defect, based on the calculated positional relationship, wherein the reference hole is defined by a first sidewall located at the same side as the incidence portion of the detector, and a second sidewall opposed to the first sidewall, and an inclination of a plane linking a bottom end and an upper end of the first sidewall is more gentle than that of a plane linking a bottom end and an upper end of the second side wall.